drm/amd/display: Expand documentation for timing

The timing programming inside DCN is far from trivial, it has multiple
parameters associated with that, and the lack of documentation does not
help comprehend this already complicated topic. This commit tries to
improve this situation by expanding the documentation of dc_crtc_timing
and the VTG program function.

Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>